In March, as filmmaker and undersea explorer James Cameronrecently descended nearly seven miles into the Challenger Deep section of the south Pacific’s Mariana Trench in a submersible of his own design, a retired Navy officer named Don Walsh waited on the surface in Cameron’s support ship. Of all the people who were part of Cameron’s team that day, Walsh was the one who perhaps best understood the difficulties and dangers that Cameron faced in visiting the deepest slot in the planet’s oceans, and making it back alive.

That’s because Walsh—almost like the fictional Rose Dawson Calvert, who accompanied the treasure hunting deep-sea divers in Cameron’s 1997 film Titanic—was the only one who actually had been there, long ago.

Back in 1960, as a young U.S. Navy Lieutenant, Walsh had piloted the Trieste—a miniature submersible designed by the late Swiss engineer Jacques Piccard, who also went along-–in the first and only previous voyage to the bottom of the trench. (From the August 1960 issue of National Geographic, here’s an account authored by Piccard of that descent.)

Walsh, in a phone interview from his Oregon home, says he was delighted to see someone finally revisit the Challenger Deep, though he seems a little disappointed that it took so long. “Back then, after Jacques and I had resurfaced and were waiting to be picked up, we started speculating about when somebody would go down there again,” he recalls. “We came to the conclusion that it would take maybe two years. Instead, it took 52.” He admits that before Cameron invited him down to his Malibu office to discuss the monumental undertaking, “I was figuring that it wasn’t going to happen again.”

After achieving international fame with the Trieste, Walsh went on to have an illustrious career as an undersea explorer, marine researcher and educator, and as founder of a private-sector consulting company, International Maritime, Inc. (He actually dived on the wreck of the Titanic, among other achievements.) But surprisingly, over the past half-century, few other explorers have ventured into the deep spots in the world’s oceans. Walsh says it’s a simple matter of cost-benefit analysis. “If you dive to 20,000 feet, which is about half of the maximum depth of the oceans, you can visit 98 percent of the sea floor,” he explains. “Engineering the machine that you need to use is a lot easier, and the cost is a lot less.”

Even so, the deepest spots in the ocean are worth visiting, says Walsh, who in retirement continues to champion undersea exploration and research. These areas may contain a potential treasure trove of data critical to understanding how the Earth’s dynamic geologic processes work—and perhaps even the secrets of how life began on our planet.

“If you’re interested in geology and plate tectonics, you probably know that sea floor is actually being created in the mid-ocean ridges,” Walsh explains. “But since the planet isn’t swelling up like a balloon being inflated, that means that someplace, sea floor is being destroyed, as well.” That destruction—tectonic plates being forced back into the Earth and recycled–occurs in the ocean’s trenches, he says. Basically, the ocean floor is “a big conveyor belt,” which completes its cycle over about a 200 million-year period.

“For a long time, this was one of the great mysteries of geology,” Walsh says. “You had rocks on the surface that were nearly as old as the planet itself, but on the sea floor, you couldn’t find rocks that were older than 200 million years. That’s because it was slowly, continuously being created and destroyed.”

In addition to gaining better understanding of that process, Walsh says that much is to be learned from studying microbial life forms that somehow manage to survive and propagate amid extremes of pressure and temperature. As this 2005 National Geographic News article details, scientists have found that soil taken from the Challenger Deep is packed with single-celled organisms called foraminifera, which are thought to resemble some of the Earth’s earliest life forms. They theorize that these life forms adapted to steady increases in pressure as the Challenger Deep formed and deepened over the last six to nine million years.

While most of the research can be performed by robotic vehicles, it’s also important to perfect advanced submersibles that allow scientists actually to visit the ocean depths, according to Walsh. For that reason, he was pleased back in 2003 when Cameron invited him to his Malibu office to discuss a plan to build the state-of-the-art craft that the filmmaker used to descend into the Mariana Trench. “We spent a whole day talking about it,” he recalls.

The technological advances incorporated into Cameron’s submersible make the Trieste look primitive, Walsh says. “It’s like resurrecting Orville Wright and taking him to a Boeing factory to show him a 747,” he quips. While the Trieste depended upon gasoline to give it buoyancy, for example, Cameron’s submarine utilizes syntactic foam, which is lighter, more efficient and won’t explode. The 2012 craft is equipped with powerful computers and digital video equipment that would have been a science-fiction fantasy in 1960. Perhaps even more important is the availability of lithium batteries, which provide the new submersible with a much more ample and longer-lasting supply of power.

“When Jim came up, it took him just 70 minutes to get to the surface,” Walsh marvels. “It took me 180 minutes, and I was going as fast as I could.”

Despite Cameron’s primary fame as maker of some of the most fantastically successful movies in history—Avatar, Titanic, and the first two films in the Terminator series–Walsh sees him as a fellow engineer and science-based explorer, rather than as a celebrity adventurer. “He’s a brilliant engineer,” Walsh says. “And he has an excellent grasp of science. During the science briefings prior to his dive, he was asking a lot of intelligent questions, ones that made it clear that he’d done a lot of reading.” Additionally, Walsh says, “Jim is an explorer. He’s creating knowledge. He doesn’t get enough credit for that.”

Just as Walsh’s job in 1960 was to test the Trieste for use as a Navy research platform, he explains that Cameron’s recent dive mostly was a “proof of concept” demonstration to validate his submersible design. Phase two of Cameron’s project will further promote scientific research. “It’s just like with the 747,” Walsh says. “You don’t just take the prototype out to the airport and start loading up passengers. You need to show that the sub is reliable and safe before you can tool up to do science.”

Walsh appreciates that Cameron invited him along as an observer on his recent Mariana Trench dive. “I was there mostly for ceremonial purposes,” he says. “Jim and his team were the ones working hard, with their shoulders to the wheel. But I was happy to be a witness to history once again.”

Don’t miss the premiere of a new half-hour special chronicling Cameron’s historic one-man dive, James Cameron: Voyage to the Bottom of the Earth, Sunday, April 29, at 9PM and 9:30PM et/pt, only on the National Geographic Channel. http://deepseachallenge.com/